1. Field of the Invention
This invention relates in general to submersible well pumps, and in particular to devices for connecting and fastening shaft elements and other portions of submersible pump assemblies.
2. Description of Prior Art
Electrical submersible pump (“ESP”) assemblies for pumping fluid from deep wells are typically made up of a series of interconnectable modular components including a motor, a seal section, and one or more pump sections with an associated fluid intake. One type of pump is a centrifugal pump made up of a large number of impellers and diffusers. Another type is a progressive cavity pump, which comprises a helical rotor rotated within an elastomeric stator having helical cavities. Each of the sections of these pumps includes an outer radial housing and interior shaft elements. The shaft elements of the different adjacent sections are connected to one another in coupling assemblies by some connection means. An example of connection means would be a set of matingly engaged splines.
During conventional ESP operation, the motor section drives the various shaft elements as well fluid is discharged to the ground surface. The shaft elements may be in clockwise rotation and the direction of thrust is downward, thus creating a compression load that is transmitted between the shaft elements. As a result of this compression, the splined connections between the shaft elements are forced together, keeping the connections intact. Thrust bearings in the seal section contain the downward thrust.
However, in situations where an ESP is operated in reverse rotation, the direction of thrust within the pump assembly is upward. In this situation, the shaft elements tend to move upward as well, creating a tension load. In a progressing cavity pump, particularly, this can cause the splined connections between the shaft elements to separate and become disengaged. Installing a physical stop element at the pump discharge can prevent this disengagement. However, stops present a significant drawback, as the placement of the stop must be matched in each individual ESP system, the weld integrity is critical, the skills involved in welding the stop must be duplicated at satellite locations, and the amount of upthrust is limited.